A few days ago, Google decided to update my Google Apps mail. Until then KCheckGmail worked just fine, but not anymore.
After looking a bit into the issue, I found that using a modified version (with the "continue" value changed) of gGMailLoginPOSTFormat as gGAP4DLoginPOSTFormat instead of the supplied one made it all work again.
I don't have access to any Google Apps domain that isn't updated (not sure if all domains are updated, but they might be, since the official Gmail blog mentioned something about it), so I'm not sure if this fix works for anyone. In other words, you should perhaps try to make a failback instead of just applying this (quick-fix) patch right away.